Dew point locus

The condition at which the liquid just begins to form is called the dew point. The condition at which the vapor just begins to form is called the bubble point. A curve can be plotted showing the temperature and pressure at which a mixture just begins to liquefy. Such a curve is called a dew-point curve or dew-point locus. A similar curve can be constructed for the bubble point. The phase envelope is the combined loci of the bubble and dew points, which intersect at a critical point. The phase envelope maps out the regions where the various phases exist. 

Of the three binary systems SCF-OS, SCF-HC and OS-HC, the first is the most relevant to understanding the principle at the base of the process. In Figure 2.3-2(a) the vapour-liquid equilibrium curves for the system CO2-toluene is shown at T = 311 K. The liquid phase is represented by the boiling point locus, the vapor phase by the dew point locus experimental data are also reported in the figure. It is clear that ... 

A simple pressure-temperature projection of the pressure-temperature-composition diagram for a mixture is given in Figure 2. It is necessary to define the terms bubble point, dew point, maxcondentherm, and maxcondenbar. A bubble point is a state of liquid mixture at which, if the pressure is decreased slightly, a second phase, a vapour, appears. Similarly, a dew point is a state of a vapour at which, if the pressure is increased slightly, a liquid phase appears. The dew point locus and the bubble point locus are continuous curves meeting at the critical point. The maxcondentherm and maxcondenbar are the maximum temperature and maximum pressure respectively on the bubble point-dew point... 

Figure 2 Bubble-point dew-point locus of a mixture of constant composition. AA and BB are paths along which retrograde condensation and retrograde evaporation take place respectively, C, critical point, D, maxcondentherm-, andE, maxcondenbar...

Figure 2-10 shows a more nearly complete pressure-volume diagram.2 The dashed line shows the locus of all bubble points and dew points. The area within the dashed line indicates conditions for which liquid and gas coexist. Often this area is called the saturation envelope. The bubble-point line and dew-point line coincide at the critical point. Notice that the isotherm at the critical temperature shows a point of horizontal inflection as it passes through the critical pressure. 

The bubble-point line is also the locus of compositions of the liquid when two phases are present. The dew-point line is tire locus of compositions of the gas when gas and liquid are in equilibrium. The line which ties the composition of the liquid with the composition of gas in equilibrium is known as an equilibrium tie-line. Tie-lines are always horizontal for two-component mixtures. 

As the pressure is further reduced, the amount of vapor increases and the amount of liquid decreases, with the states of the two phases following paths b c and he1, respectively. The dotted line from b to c represents the overall states of the two-phase system. Finally, as point c is approached, the liquid phase, represented by point c, has almost disappeared, with only minute drops (dew) remaining. Point c is therefore called a dew point, and the P - yt line is the locus of dew points. Once the dew has evaporated, only saturated vapor at point c remains, and further pressure reduction leads to superheated vapor at point d. 

Curve ABC in each figure represents the states of saturated-liquid mixtures it is called the bubble-point curve because it is the locus of bubble points in the temperature-composition diagram. Curve ADC represents the states of saturated vapor it is called the dewpoint curve because it is the locus of the dew points. The bubble- and dew-point curves converge at the two ends, which represent the saturation points of the two pure components. Thus in Fig. 3.6, point A corresponds to the boiling point of toluene at 133.3 kPa, and point C corresponds to the boiling point of benzene. Similarly, in Fig. 3.7, point A corresponds to the vapor pressure of toluene at 100°C, and point C corresponds to the vapor pressure of benzene. 

Figure 3.2 shows a phase envelope for an acid gas mixture. Note that the locus at lower pressure is the dew-point curve, whereas the one at higher pressure is the bubble-point curve. In fact, any point inside the phase envelope is a two-phase point. 

This would correspond to the dew-point calculation as performed for a vapor-liquid equilibrium condition. That is, it would correspond to a point or locus of points on the saturated vapor curve, as distinguished from the saturated liquid curve. (For a single or pure component, they are one and the same.)... 

The points Ci and are the critical points of pure methane and ethane, respectively. The line connecting these two points, which is the intersection of the bubble point and dew point surfaces, is the critical locus. This is the set of critical points for the various mixtures of methane and ethane. The black curve connecting points A and Ci is the vapor pressure curve of pure methane, and the violet curve connecting points B and C2 is the vapor pressure curve of pure ethane. 

Bubble points and dew points may be generated as described above for a given mixture over ranges of temperature and pressure. The locus of bubble points is the bubble point curve and the locus of dew points is the dew point curve. The two curves together define the phase envelope. In addition to the bubble point curve (total liquid saturated) and the dew point curve (total vapor saturated), other curves may be drawn representing constant vapor mole fraction. All these curves meet at one point, the critical point, where the vapor and liquid phases lose their distinctive characteristics and merge into a single, dense phase. 

Figure 3.2c shows the familiar cigar-shaped vapor—liquid envelope found in many elementary textbooks on phase equilibria. At a fixed overall composition (denoted by x in this figure) there exists a single vapor phase at very low pressures. As the pressure is isothermally increased, the two-phase vapor-liquid envelope is intersected and a dew or liquid phase now appears. The locus of points that separates the two-phase vapor-liquid region from the one-phase vapor region is called the dew point curve. The concentration of the equilibrium vapor and liquid phases within the two-phase boundary of the vapor-liquid envelope is determined by a horizontal tie line similar to the one depicted in this figure. 

In Fig. 10.3-2 we have plotted, for various fixed compositions, the bubble and dew point pressures of this mixture as a function of temperature. The leftmost curve in this figure is the vapor pressure of pure ethane as a function of temperature, terminating in the critical point of ethane (for a pure component, the coexisting vapor and liquid are necessarily of the same composition, so the bubble and dew pressures are identical and equal to the vapor pressure). Similarly, the rightmost curve is the vapor pressure of pure propylene, terminating at the propylene critical point. The intermediate curves (loops) are the bubble and dew point curves relating temperature and pressure for various fixed compositions. Finally, there is aline in Fig. 10.3-2 connecting the critical points of the mixtures of various compositions this line is the critical locus of ethane-propylene mixtures. 

Figure 10.3-2 The bubble point, dew point, and critical locus for the ethane-propylene system.

Hence, for each component the concentration of a vapour rising from the tray ( +l) equals the concentration of the liquid flowing from the tray n. This relation defines a distillation curve as the locus of the tray compositions at total reflux. A distillation curve map (DCM) can be generated easily by choosing a tray liquid composition , and stepping up and down by a series of bubble and dew points. 

The apparatus can be also used to establish the dew point-bubble point locus. The great advantage of Kay s apparatus over most types of apparatus for determining critical properties is that it is relatively simple to operate. However, it does have the disadvantage that the samples are not degassed. This could lead to errors of 0.01 to 0.06 MPa in the critical pressure and up to 1 K in the critical temperature. 

The region in which vapor and liquid may coexist in a binary system is limited by the vapor pressure curves of the pure components and the critical line. In Figure 5.9 the vapor pressure curves of the pure compounds of the system ethane-heptane are shown together with the PT-curves of different fixed compositions of the liquid and the vapor phase. The intersections of the dew point and the bubble point curve for a given temperature and pressure mark the VLE for the chosen compositions in the liquid and the vapor phase. The critical points of a binary system can be found where a loop in Figure 5.9 is tangential to the envelope critical curve, also called critical locus. 

Between the bubble and dew point curves we have, of course, mixtures of liquid and vapor in equilibrium with each other. The dashed line in Figure 14.2, for example, represents the locus of such mixtures, where the liquid phase is 20% of the total. 

In P-T projections, the composition axis is collapsed into the pressure-temperature plane. The vapor pressure curve for component A is labeled LV(A) and that for component B is labeled LV(B). These curves terminate at the component critical points (L = V) designated as hollow circles. In Fig. 2, dew pressure and bubble pressure curves for an intermediate composition x intersect at a point on the (L = V) critical locus where the liquid and vapor phases become critically identical. Normally, dew and bubble pressure curves are not shown in projections. They are shown here so that the construction of the related P-x at fixed T, and T-x at fixed P, phase diagrams is clearly illustrated. Each critical point on the critical locus corresponds to a fixed composition. Points close to the critical point of component A are critical points for mixtures with high concentrations of A, whereas points closer to the critical point of... 

In Class B1 systems branch II of the critical locus spans the entire temperature range from the critical temperature of the heavy component (if this is accessible without decomposition) down to and below the critical temperature of the solvent, as in curves (a) and (b) in Figure 1.9. On raising the pressure at a constant temperature which is above the solvent critical temperature, complete miscibility between the liquid and supercritical fluid phases occurs at the pressure (the critical pressure) corresponding to this temperature on the locus curve. The dew- and bubble-point curves then merge giving a closed loop pressure/composition diagram. 

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